1. Field of the Invention
The present invention relates to dual mode radio frequency transmitters, and more particularly to a dual mode transmitter having a to-and-from transmission path including a switched linear amplifier responsive to a mode-of-operation control signal.
2. Description of the Background Art
U.S. Pat. No. 5,121,081 issued Jan. 9, 1992 to Hori entitled "OUTPUT WAVEFORM CONTROL CIRCUIT" discloses an output waveform control circuit for a time division multiple access system, including a second driving circuit which sends a control signal to the power amplifier in accordance with a signal outputted by the comparator circuit. Thus, by controlling the operating voltage of the power amplifier simultaneously with the control of the level input signal from the input level control circuit, the output characteristic of the power amplifier, including a class C or the like amplifier having a non-linear input/output characteristic, is prevented from varying abruptly, and the output waveform of the power amplifier is so controlled as to have gently sloped leading and trailing edges.
U.S. Pat. No. 5,060,294 issued Oct. 22, 1991 to Schwent et al. entitled "DUAL MODE POWER AMPLIFIER FOR RADIOTELEPHONE" discloses a dual mode power amplifier operable in either the linear mode or the saturation mode. The power amplifier, when comprising a portion of a radio transmitter, permits efficient amplification of either a frequency modulated or a composite modulated signal by operation of the amplifier in the saturation mode or the linear mode, respectively. A processor determines the modulation-type of the signal and generates a signal to cause operation of the amplifier in either the saturation mode or the linear mode.
U.S. Pat. No. 4,955,083 issued Sep. 4, 1990 to Phillips et al. entitled "DUAL MODE RADIO TRANSCEIVER FOR AN SSB COMMUNICATION SYSTEM" discloses a radio transceiver for use in an SSB communication system having FM data capability. The dual-mode radio transceiver of the present invention includes a receiver capable of demodulating and deriving AFC from either the voice channel having voice messages transmitted via single sideband amplitude modulation with a pilot carrier, or the data channel having high speed data messages transmitted via narrowband frequency modulation in the same channel bandwidth. Furthermore, the dual-mode radio transceiver includes a transmitter capable of transmitting one of either of the above types of modulation on the appropriate channel, as determined by information received from the high speed data messages transmitted on the control channel.
U.S. Pat. No. 4,924,191 issued May 8, 1990 to Erb et al. entitled "AMPLIFIER HAVING DIGITAL BIAS CONTROL APPARATUS" discloses an amplifier equipped with a digital bias control apparatus to provide precise, dynamic control over the operating point of a plurality of amplifying elements in the amplifier. A processor optimizes the operating point of each individual amplifying element as a function of the amplifying element characteristics, the operating environment and the applied input signal. The use of a processor also enables the user to remotely program the operating point of each individual amplifying element in the amplifier. The processor further enables dynamic changes in the operating characteristics of the amplifier as the operating environment of these amplifying elements changes. The processor also generates an alarm signal if any particular amplifying element is operating out of its nominal specifications. This digital bias control apparatus can function in class A, AB, B or C type of amplifiers whether they are tuned or untuned and whether the amplifier operates in a pulsed or continuous mode.
U.S. Pat. No. 4,446,440 issued May 1, 1984 to Bell entitled "DUAL MODE AMPLIFIER" discloses an apparatus that amplifies the signal in a linear mode when the voltage of the signal is much less than the voltage of the power source, and in a switching mode when the signal is at other voltages.
U.S. Pat. No. 4,257,009 issued Mar. 17, 1981 to Yorkanis entitled "INHIBIT CIRCUIT FOR DIFFERENTIAL AMPLIFIER" discloses a system wherein the outputs of a pair of non-additive combiners are coupled to respective inputs of a differential amplifier and an a.c. signal to be amplified is applied to a selected input of one of the combiners. A control circuit operates in a first mode to apply control voltages to the combiner inputs for causing the one combiner to block the a.c. signal and also causing both combiners to apply a common mode input voltage of a first value to the amplifier inputs. In a second mode, the control circuit changes the values of the control voltages in a sense to cause the one combiner to pass the a.c. signal and to also cause both combiners to apply a common mode input voltage of a second value to the amplifier inputs whereby, in either mode, the amplifier produces a constant quiescent d.c. output voltage.
U.S. Pat. No. 4,034,308 issued Jul. 5, 1977 to Wermuth et al. entitled "AMPLIFIER WITH CONTROLLABLE TRANSMISSION FACTOR AND SWITCHABLE CONTROL CHARACTERISTIC" discloses an amplifier circuit whose gain bears a desired relation to a control voltage, including a differential amplifier, a plurality of impedances interconnected between the amplifier terminals and switches interconnected with the impedances and switchable between two states which create two impedance configurations that give the circuit mutually complementary gain vs. control voltage control characteristics..
Japanese Patent 54-104760 issued Aug. 17, 1979 to Hikari Honda entitled "AMPLIFIER OF LOW POWER CONSUMPTION" discloses a system a bias control signal circuit for an amplifier to provide lower power consumption. The output power is detected and a bias control signal is produced that changes the operation point of the amplifier depending on the transmission signal level.
European Patent Application No. 90302627.6 filed Mar. 13,, 1990 by Vaisanen and published Oct. 3, 1990, entitled "A METHOD FOR CHANGING THE OPERATING CLASS OF A TRANSMITTER" discloses a method for changing the operating class of a radio telephone transmitter used especially in a gsm radio telephone system. When the transmitter comprises one or several power stages of class A, B, AB or C, it is possible according to the invention to change the operating point of one or several power stage transistors by means of a controlled semiconductor switch whereby the class of the power stage amplifier changes to that desired.